Instead of new languages what I'm hoping for is addons/improvements to already existing languages (like the original 5) with improvements like more sentence variety, more accepted translations, and even new skills added on if they can so we can get further into a language using DL alone. My bets are on Norwegian, Indonesian and/or Greek.

There's not really any reason for new courses to be put on hold so that existing courses can be improved. The volunteer course contributor teams work independently of one another. It's not as if Norwegian experts are going to be able to help improve Italian course. Unlike new courses, existing courses take little in the way of developer involvement. This is why Duolingo constrains the number of new courses in Phase 1 at any given time, not because of a lack of qualified, volunteer language experts.

For English speakers, probably Norwegian, Finnish or Czech if the team isn't too busy with the reverse course. For learners of English, probably Hebrew or Farsi. And for courses between existing languages, Spanish for Italian or Dutch <-> French has to be one of the next ones.

Improvements to existing courses would be good. 10 quality courses is better than 100 buggy ones. But also, phase two has 22 courses, twice what phase one has. Duo might focus on stabilizing those before adding new to the mix.

Salam. I thought the same thing about Ki Swahili, and agreed with KB in wishing for more add ons/improvements to existing languages. Spanish for English speakers would be cool with stuff like por/para, irregular verbs activities; types-of-Spanish comparison maybe through better song libraries, lyrics translation challenges; culture immersion discussions, more differences and likes comparisons....

I'm hoping for Finnish, Norwegian and Icelandic so we'll have all the languages of the Nordic countries! The former two especially seem logical choices, and there is evidently demand for them.
Apart from that, I'm hoping for more Celtic languages (I'd love Welsh and Breton!), Greek, Latin, and maybe some non-Indo-European ones (Basque would be interesting)?
Improving existing courses is also a good idea, or possibly adding to the tree (Advanced French/German/Spanish/..?)

I don't have a particular interest in learning Finnish, but if the great writer Tolkien talks about Finnish as if "entering a complete wine-cellar filled with bottles of an amazing wine of a kind and flavour never tasted before" it must be so :P

Theoretically, there's not much of a limit in terms of what the system can handle, but there are a few very important practical limits. First, new courses still require significant staff involvement to work through issues that arise with each course, and as we all know, Duo has finite staff resources. Additionally, each new language requires either licensing a TTS engine to use with the course or paying voice actors to produce quality recordings (something the Irish course is still struggling with, apparently). Both of these options have a considerable expense associated with them, so Duo has to give some thought to how they prioritize which languages they'll add. That's why I'll be relatively surprised if we see more "low utility" languages like Irish or Esperanto added in the near future. Finally, they constrain the number of new courses to ensure that what's being produced is of decent quality. To paraphrase something I heard an artisanal salami maker say a few years ago, you can only make so much salami and still do it well.

If there is ever a BENGALI course for English speakers, I think I might be able to contribute a little (https://en.wikipedia.org/wiki/Bengali_language). It is the sixth largest language in the world in terms of number of speakers.